To concentrate a highly heat-sensitive liquid, it is necessary to effect concentration in a thermal contact state at low temperature and in a short period of time. To this end, it is required that a raw liquid supplied to a concentrator be withdrawn in one pass as a concentrate. In the case of withdrawal in one pass, the amount of liquid decreases with concentration; thus, how to uniformly distribute this decreasing liquid over a heat transfer surface is an important point. Heretofore, a concentrator which is a long-tube version of the heat transfer tube system has been used. With this system, however, a uniform distribution of raw liquid over a heat transfer surface can hardly be attained; for example, there has been a drawback that the flow of raw liquid deviates to one side of the heat transfer tube, considerably decreasing the heat transfer efficiency. Another drawback is that cleaning is not easy.
A concentrator based on the heat transfer plate system is also known. In this system also, the distribution of raw liquid is insufficient and the plate is short in length for its width; therefore, in the case where the amount of liquid is small, the plate surface dries, resulting in the scorching of the raw liquid, leading to a deterioration in quality.
To remedy the above drawbacks, we have proposed the following.
We have proposed an apparatus which operates on the principle of forming a preheating channel along the longitudinal center line of a heat transfer plate in which a raw liquid ascends, while forming heating channels on both sides of said preheating channel in which the raw liquid flows down in thin film form, preheating the raw liquid while the latter is ascending, distributing the raw liquid to both sides from the upper end of said preheating channel, and heating the raw liquid during this flow so as to evaporate and separate the moisture, thereby providing a concentrated liquid (see Japanese Patent Application Laid-Open Specification No. 22990/1987, dated Jan. 31, 1987, published by the Japanese Patent Office).
The above proposal is capable of contributing to the remedy of the drawbacks involved in using the conventional concentrator based on the heat transfer plate system, but much remains to be improved in connection with improving the performance in such regions in the heat transfer plate as the preheating channel, distributing region and heating channels.
This invention has been proposed in view of the above problems in the prior art, and an object thereof is to improve the performance in such regions in the heat transfer plate as the preheating channel, distributing region and heating channels.